1. Field of the Invention
This invention relates to an optical disc device and an optical pickup device for recording and/or reproducing information signals on or from a disc-shaped recording medium such as an optical disc, a magneto-optical disc or a phase-change disc.
2. Description of the Related Art
There has hitherto been known an optical pickup device for reproducing information signals from, for example, an optical disc. This type of the optical pickup device has an objective lens 53 for focussing a laser light beam on an information recording surface of an optical disc 51, as shown in FIG. 1. The objective lens 53 is held in a substantially cylindrically-shaped lens holder 54, as shown in FIG. 1. The inner rim of the lens holder 54 is formed with an aperture portion 55 facing the light incident surface of the objective lens 53 for throttling the laser light incident on the objective lens 53.
With recent yearning towards a higher order of the information, there is a demand for a higher capacity of information signals for an optical disc 51. For realizing the high surface density of the optical disc 51, it is necessary to reduce the spot diameter of the laser light. For reducing the spot diameter, it is necessary not only to shorten the wavelength of the laser light but also to increase the numerical aperture NA of the objective lens.
If, with the single lens type objective lens, it is attempted to increase the numerical aperture NA, it has so far been difficult to realize a pre-set non-spherical surface by a non-spherical coefficient because of the large refractive power of the single lens. With this in view, a double-lens type objective lens provided with two lenses having the optical axes coincident with each other has been proposed in U.S. Pat. No. 5,125,750. On the other hand, the technique on a double-lens type objective lens unit 56 has also been proposed in Japanese Laying-Open publication H-9-251645 (U.S. Ser. No. 08/811,727) and in Japanese Laying-Open publication H-9-251662 (U.S. Ser. No. 08/813,364).
The double-lens type objective lens unit 56, in which attempts have been made towards increasing the numerical aperture NA, is explained by referring to the drawings. The double-lens type objective lens unit 56 has a first lens 57 (forward lens) arranged facing the optical disc 52 and a second lens 58 (backward lens) arranged coaxially with the first lens 57, as shown in FIG. 2.
The double-lens type objective lens unit 56 also includes a substantially cylindrically-shaped lens holder 59 for holding the forward lens 57 and the backward lens 58, and an aperture 60 arranged on the inner rim of the lens holder 59 for throttling the laser light incident on the backward lens 58, as shown in FIG. 2.
Meanwhile, the double-lens type objective lens unit with the increased recording capacity has an information recording surface 52b closer to the forward lens 57, with the interposition of a transparent substrate 52a. This optical disc 52 has a drawback that spherical aberration is apt to be produced with an error in thickness of the transparent substrate 52a.
With this in view, it has also been proposed to provide an electromagnetic actuator, not shown, in order to adjust the separation along the optical axis (indicated by X in FIG. 2) between the forward tens and the backward lens for reducing the spherical aberration ascribable to thickness of the transparent substrate 52a of the optical disc 52.
However, the electromagnetic actuator provided in the double-lens type objective lens unit includes a magnet, a coil or a yoke to provide a magnetic circuit, so that it is rather complex in structure, Thus, the entire optical pickup device is increased in size.
In addition, with the above optical pickup device, an electrical voltage needs to be applied at all times across the electromagnetic actuator in order to maintain a distance between the forward and backward lenses.